1. Field of the Invention
The present invention relates generally to a bright tin-cobalt plating bath and more particularly to a bright tin-cobalt plating bath with an additive as a brightener.
2. Description of the Prior Art
In general, electroplated coatings of tin alloys have superior anti-corrosive characteristics. Tin-nickel alloy plating is one of the tin alloys that has been used and is superior in its anti-corrosive characteristic to a plating of tin or nickel alone. The tin-nickel alloy plating shows a reddish appearance. In order to carry out the tin-nickel alloy plating, an acidic fluoride bath is generally used, and the electroplated layer thus formed is very brittle and a crack is apt to be formed in the layer when stress is applied to it.
A pyrophosphate bath has been also proposed for the tin-nickel alloy plating but the thus formed electroplated layer is also brittle and it is more difficult to control the pyrophosphate bath than it is to control the acidic fluoride bath. For this reason, the pyrophosphate bath is not used in practice.
Tin-cobalt alloy plating is also known to avoid the undesirable brittleness of the tin-nickel alloy plating. The tin-cobalt alloy plating is substantially equal to the tin-nickel alloy plating in its anti-corrosive characteristic but is substantially better in its brittleness. In addition, cracks are not apt to form in the tin-cobalt alloy plating. As a practical plating bath, an acidic fluoride bath is used and the thus formed plating has a color equal to that of chromium plating. Therefore, the tin-cobalt alloy plating may be employed as a finishing plating in place of the chromium plating which is usually employed.
However, the tin-cobalt alloy plating bath including fluoride requires difficult drainage and exhaust treatments so that such a bath is undesirable from the point of view of environmental pollution.
A report (Electrodeposition of Alloys. Vol. 2, Academic Press, New York and London, pp. 339 - 341, 1963 edited by A. Brenner) has been issued by V. Sree and T. L. Rama Char on tin-cobalt alloy plating from a pyrophosphate bath. The inventors of the present invention have carried out a test based upon such report and in which ammonium citrate is added to the pyrophosphate bath as an additive, as disclosed in the report. Throughout this test, the contents of the bath were agitated to carry out a so-called Hull Cell Test. The electroplated coating formed by this test had black or grey blurs.